The invention relates generally to lightwave communication systems and, more particularly, to gain control for optical amplifiers used in such lightwave communication systems.
To meet the increasing demands for more bandwidth and higher data rates in today""s networks, wavelength division multiplexing (WDM) is being used extensively in long haul optical transmission systems and is being contemplated for use in short haul applications, such as metropolitan area networks and the like. As is well known, WDM combines many optical channels each at a different wavelength for simultaneous transmission as a composite optical signal in a single optical fiber.
Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. Rare earth-doped fiber optical amplifiers, such as erbium-doped fiber amplifiers, are predominantly used in WDM systems, although other types of optical amplifiers such as semiconductor optical amplifiers may also find use in such systems.
In an optically amplified VDM system, signal power excursions in the WDM signal can be a significant problem. Signal power excursions may arise as a result of adding or dropping optical channels, network reconfigurations, failures or recovery from failures, and so on. As used hereinafter, surviving optical channels are meant to refer to those optical channels that are still present in the WDM signal after an add/drop has occurred. For example, adding or dropping individual channels of a WDM signal may cause changes in input power, which in turn results in changes in gain as well as fluctuations of power levels in surviving optical channels. Stated otherwise, because the output power of an optical amplifier does not react accordingly to the changes in input power, the optical power per surviving channel will fluctuate. Using an uncontrolled optical amplifier as an example, when 4 out of 8 channels in a WDM signal are dropped, the power in each surviving channel then increases toward double its original channel power in order to conserve the saturated amplifier output power. This increased gain per channel and increase in power per channel can lead to transmission stabilization problems, unacceptable bit error ratio degradation if power variations are not within the dynamic range of receiver detection equipment, as well as other power-related problems. For example, surviving channels may experience errors when channels are dropped because the power in the surviving channels may exceed thresholds for nonlinear effects, such as Brillouin scattering. Surviving channels may also experience errors when channels are added, thus leading to optical signal to noise ratio (OSNR) degradation or even more severe impairments if power in surviving channels is depressed below the sensitivity thresholds at the receiver.
Additionally, because gain of an optical amplifier cannot be controlled fast enough in prior control schemes in response to changes in input power, power spikes may occur in the total output power of the optical amplifier. Power spikes will also occur in the total output power of an uncontrolled optical amplifier as well. These power spikes can adversely affect system performance, e.g., by degrading bit error ratio performance, by damaging receiver components if the power levels exceed thresholds, and so on. As can be expected, changes in input power and resulting gain fluctuations are especially problematic for systems in which a large amount of traffic is added and dropped, e.g., metropolitan area networks, systems employing wavelength add/drop multiplexers or optical cross connects, and so on.
Many different gain control schemes have been proposed for controlling signal power excursions or transients. Some gain control schemes employ a feedback loop to control the amount of pump power supplied to the optical amplifier based on measurements of the total output power of the optical amplifier. However, this method of gain control is not fast enough to respond to the sudden changes in power at the input of the optical amplifier. Similarly, some have proposed feed-forward compensation using a low-frequency control loop as well as software-based gain control schemes. In each of the cases, a gain control scheme has not yet been demonstrated which has fast enough response times for limiting surviving channel power excursions as a function of the input power variations. Gain clamping is another well-known technique, but inefficient pump power usage is a known problem with gain-clamped optical amplifiers.
In an optically amplified wavelength division multiplexed (WDM) system having a WDM signal comprising a plurality of optical channels, the per-channel gain of the optical channels is kept relatively constant despite changes in input power at the optical amplifier, such as when individual optical channels of the WDM signal are added and dropped. More specifically, gain of an optical amplifier is controlled in a feed-forward based control scheme by controlling the amount of pump power supplied to the optical amplifier as a function of changes in measured optical input power which are measured in a feed-forward monitoring path. The amount of pump power for effecting gain control is adjusted according to a scaled relationship to the measured input power of the optical amplifier. By controlling the pump power directly in response to changes in input power, gain of the optical amplifier in one exemplary embodiment can be controlled on a sub-microsecond time scale from the time that a change in input power is detected. As such, gain control can be effected before changes in input power reach the gain medium of the optical amplifier. Moreover, by maintaining relatively constant per-channel gain in an amplified WDM signal despite changes in input power at the optical amplifier, power excursions are substantially reduced in surviving optical channels of the WDM signal, i.e., those at the output of the optical amplifier.
In one illustrative embodiment, a WDM system includes at least one erbium-doped optical amplifier for amplifying a WDM signal having a plurality of optical channels. The optical amplifier is coupled to and receives pump light from a pump source. At a position upstream from the optical amplifier input, the WDM signal is coupled via a feed-forward monitoring path to an optical monitoring arrangement which detects and measures the total input power of the WDM signal. In response to fast changes in input power (e.g., add/drop, failure, etc.), control circuitry coupled to the pump source controls the amount of pump power being supplied to the optical amplifier. As a result, gain is controlled before changes in input power reach the optical amplifier gain medium such that power levels of surviving optical channels will experience minimal power excursions despite changes in input power. For example, the power of a surviving channel at the output of the optical amplifier is relatively constant regardless of how many channels and how much power is supplied at the input of the optical amplifier. Of course, input power must remain within certain boundaries for reasons relating to device and system stability, physics, and so on.
Fast gain control can be achieved according to the principles of the invention when new optical channels are added to or dropped from the WDM signal, in the presence of failures or recovery from failures, e.g., transmitter failure, when channels are re-routed such as in cross-connects, and so on.